This invention relates to a gas sensor of the type having a gas diffusion chamber defined in a narrow gap between two platy solid electrolyte cells and provided with gas diffusion restricting means.
Many types of gas sensors using solid electrolytes have been developed and put into practical use for various industrial purposes and also for antipollution purposes.
A relatively recently developed gas sensor is characterized in that a plate-like solid electrolyte cell using a selected solid electrolyte which is conductive to ions of a specific gas and having a pair of electrode layers on the opposite surfaces and another solid electrolyte cell of substantially the same construction are held opposite and parallel to each other so as to define a gas chamber, which admits a gas atmosphere subject to measurement, between the two cells and that the gas chamber is provided with appropriate gas diffusion restricting means. One solid electrolyte cell is used as a concentration cell element and the other cell as an ion pump element. Usually the current for operation of the ion pump cell element is controlled so as to keep the output voltage of the concentration cell element at a predetermined constant level, and the actual value of the pump current is detected as an output signal indicative of the concentration of the specific gas in the gas atmosphere in which the gas sensor is used. For example, U.S. Pat. No. 4,505,806 issued to Yamada discloses an oxygen sensor of this type.
In such a gas sensor the correlation between the output of the sensor and the concentration of the gas to be sensed depends on the effects of the gas diffusion restricting means provided to the gas chamber and also on a gas diffusion restricting effect of the gas chamber itself, which is attributed to thinness of the gas chamber or narrowness of the gap between the two solid electrolyte cells. The effects of the gas diffusion restricting means are determined at the manufacturing stage and can be adjusted even after completion of the manufacturing process. However, the thickness of the chamber cannot be adjusted after the manufacturing process. Furthermore, in industrial manufacture of a large number of gas sensors it is not easy to make the thickness of the gas chamber constant and uniform because of several reasons including some shrinkage or distortion of the solid electrolyte plates at the firing step in the manufacturing process.